(1) Field of the Invention
The present invention relates to a method for starting drawing of a GRIN lens fiber from a preform in a method for drawing the GRIN lens fiber, and in particular, relates to a method for reducing time taken to start drawing and decreasing an amount of wasted preform.
(2) Description of Related Art
A so-called GRIN fiber lens is a cylindrical graded index lens (GRaded INdex lens) having a refraction index distribution within a glass so as to bend a traveling direction of light, thereby realizing the function of the lens. Typically, the GRIN lens is cut into pieces of less than about 1 mm, each of which is welded to the tip of an optical fiber, and used in a variety of applications in optical systems for optical communications. Today, there is a demand for GRIN lenses having high NA (numerical aperture). Preforms of such GRIN lenses are fabricated in a sol-gel method as disclosed in Patent Document 1 below and the like. A preform 7 that is fabricated using the sol-gel method has a cylindrical shape whose diameter and length are respectively about 5 mm and about 30 to 50 mm at the minimum. This size is very small compared to that of a silica glass preform for an ordinary optical fiber.
FIG. 5 illustrates an example of a related-art for starting drawing of the GRIN lens fiber. In FIG. 5, a diagram on the left illustrates a state in which the preform 7 is set in a heater 2 of a heating furnace. At this time, the preform 7 is disposed such that the distance between a maximum temperature portion (a center of a heating portion of the heater 2) of the heating furnace and a lower end of the preform 7 is about 12 to 18 mm. The diagrams in the center and on the right illustrate a state in which the preform 7 is heated and starts to soften. A preform lower end portion 7a (part of the preform below the maximum temperature portion of the heater) starts to descend due to its own weight.
Patent Documents 2 and 3 below and the like disclose a technology for starting drawing of ordinary optical fiber preforms.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2005-115097    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2005-47754    Patent Document 3: Japanese Unexamined Patent Application Publication No. 2008-120643
With the related-art method for starting drawing of the GRIN lens preform, 12 to 18 mm of the preform from the bottom is softened and elongated. Thus, at least about 15 to 21 mm (about ⅓ to ½ of the length of the preform) of the preform is not drawn and wasted. Since the GRIN lens preform is expensive and valuable, this is a significant loss in terms of the cost.
In addition, the preform having the length of 12 to 18 mm weighs about 0.5 to 0.8 g. Thus, a descending speed of the softened part is very slow, and accordingly, it requires a very long time before winding is started. In order to reduce the time taken before winding is started, a method in which the heating temperature is increased in order to decrease the viscosity of the preform seems useful. However, since the silica glass of the GRIN lens includes a dopant (such as titanium or tantalum that causes the GRIN lens to have a refraction index distribution), a change in viscosity relative to a temperature change is comparatively large. This may cause a fused and elongated portion to be cut or to become too thin to be wound up during heating, thereby wastage of the preform being increased.
The technology disclosed in the above-described Patent Documents 2 and 3 is used to start drawing of an ordinary optical fiber, and is not useful at all when drawing the GRIN lens preform, which is very small compared to the size of the preform of the optical fiber.